splitmuxsrc: make sure to push sticky events before adding pad
splitmuxsrc: make sure to push sticky events before adding pad

It allows the caps to be set on the pad before being added for
dynamic autoplugging to work.

https://bugzilla.gnome.org/show_bug.cgi?id=751240

sebras/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,shelsonjava/gst-plugins-good,froggatt/gst-plugins-good-m,shelsonjava/gst-plugins-good,froggatt/gst-plugins-good-m,chamois94/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,froggatt/gst-plugins-good-m,rawoul/gst-plugins-good,vatavuserban/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,shelsonjava/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,kittee/gst-plugins-good,vatavuserban/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,shelsonjava/gst-plugins-good,sebras/gst-plugins-good,sebras/gst-plugins-good,ikonst/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,chamois94/gst-plugins-good,StreamUtils/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,froggatt/gst-plugins-good-m,pexip/gst-plugins-good,kittee/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,ikonst/gst-plugins-good,sebras/gst-plugins-good